The present invention refers to an improvement introduced in a starting system for an induction electric motor, particularly a starting system of the type installed inside the shell of a hermetic compressor of refrigeration, such as described in the co-pending patent applications of the same applicant and filed on Apr. 13, 1998 and Oct. 2, 1998.
In the conventional constructions, an electric motor of the type mentioned above has a main winding and an auxiliary winding, which are supplied with electric current from an electric current source through a starting circuit of the electric motor usually comprising a starting relay or a thermistor (PTC), mounted to a casing to be electrically connected to the external wall of the shell of the compressor, in which this electric motor operates.
In these constructions, a terminal of each of the main and auxiliary windings of this electric motor is directly connected to the electric current source, through a switching element of this starting circuit, whereas another terminal of the auxiliary winding is electrically connected to said electric current source, through the starting relay or PTC.
The construction using a starting relay has, as disadvantages, the possibility of the relay contacts be closed in undesirable occasions or be fused in a situation of high discharge of electrical interference currents, and the occurrence of sparks.
The construction using a starting PTC does not avoid that, in practice, the resistance of the PTC is incapable of effectively interrupting the current supply to the auxiliary winding, allowing a residual current to remain through the latter, resulting in energetic consumption of the electric motor. Besides, the existence of a current flowing through the PTC keeps it heated, which makes difficult new starts of the electric motor, since a new start will only occur when the PTC presents a determined temperature in which there is no significant resistance to the passage of electric current.
Besides the inconveniences resulting from the provision of a starting relay or a PTC for obtaining the start of the electric motor, this conventional of providing the starting circuit with such elements externally to the compressor causes some inconveniences, such as difficulty and discomfort in mounting said casing to the compressor and the exposure thereof to impacts upon displacement of the compressor, for example, during transportation, which may result in an inadvertent disconnection of the casing containing the starting circuit from the compressor shell.
Aiming at dispensing the use of a starting relay or a starting PTC in the starting circuit of the motors considered herein, in order to avoid the inconveniences resulting from the utilization of said components, there has been proposed the solution defined in the co-pending patent application, filed on Oct. 2, 1998, claiming the national priority of the prior patent application PI 9801913.9 (abandoned), according to which the starting system comprises: a switching means provided between the electric current source and the auxiliary winding; and an actuating means provided inside the electric motor and subject to a magnetic field produced by the main winding upon energization thereof for operation of the electric motor, said actuating means closing the switching means, in order to energize the auxiliary winding, when the starting current passing through the main winding produces a magnetic field of a certain intensity, and opening the switching means, in order to de-energize the auxiliary winding, when the normal operation current through the main winding produces a magnetic field with less intensity.
This starting system mounted inside the compressor shell is highly advantageous, because it eliminates the inconveniences mentioned above related to the use of the starting relay and PTC. However, since the switching means is positioned inside the compressor shell, in case said element fails, after the compressor has been installed in a refrigeration system, the repair operation will require the complete substitution of the compressor, which is a long and costly procedure.
Thus, it is a general objective of the present invention to provide an improvement in a starting system for an electric motor of the type installed inside the shell of a hermetic compressor and which may be easily and rapidly discarded in operational terms and substituted by a starting system, which may be attached to the outside of the compressor shell, with no need of substituting said compressor.
The present improvement is applied to a starting system for an electric motor, which drives a hermetic compressor of refrigeration and comprises a main winding, which is energized by an electric current source; and an auxiliary winding, having a terminal directly connected to the electric current source and the opposite terminal connected to the electric current source through a starting device mounted to the compressor, inside the shell thereof, the main winding and the auxiliary winding being connected to the electric current source by means of contact terminals of a hermetic terminal affixed to the outside of the compressor shell.
According to the present invention, the starting device is mounted in series with a fusible element, to be opened when submitted to an electric current of a certain intensity, in order to definitively interrupt the energization of the auxiliary winding by means of the starting device, said hermetic terminal being designed to receive an external starting device, in order to promote the selective connection between said other terminal of the auxiliary winding and the electric current source upon the definitive opening of the circuit defined by the fusible element and the starting device.
The provision of the fusible element in series with the starting device internal to the compressor shell allows, in case said starting device fails, an electric current to be applied trough said starting device and said fusible element, with an intensity superior to that of the operational and starting levels of the compressor, but sufficient to provoke the definitive opening of the fusible element and the operational discard of the starting device mounted in series with said fusible element. With the operational discard of the starting device inside the compressor shell, another starting device, of any adequate construction, may be then rapidly and easily adapted to the hermetic terminal affixed outside the compressor shell, re-establishing the control of the selective connection between the auxiliary winding and the electric current source. The operational capability of the compressor may be thus re-established, with no need of replacing or even dismounting the same.